Self-destructing filter cake

ABSTRACT

A composition and method are given for self-destructive fluid loss additives and filter cakes in wellbores and subterranean formations. The fluid loss additives and filter cakes are formed from a mixture of particulate solid acid-precursors, such as a polylactic acid or a polyglycolic acid, and particulate solid acid-reactive materials, such as magnesium oxide or calcium carbonate. In the presence of water, the solid acid-precursors hydrolyze and dissolve, generating acids that then dissolve the solid acid-reactive materials. The composition is used in oilfield treatments such as drilling, completion and stimulation where it disappears when it is no longer needed without the use of mechanical means or injection of additional fluids.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 60/421,696, filed on Oct. 28, 2002.

BACKGROUND OF INVENTION

This invention relates to a composition and method for generatingself-destructing filter cakes in wellbores and in subterraneanformations. More particularly it relates to a composition and method forinjection of solids-containing fluids that form filter cakes in whichacids are generated after the filter cakes have been placed. Finally, itrelates to using the composition and method in oilfield applications.

There are many oilfield applications in which filter cakes are needed inthe wellbore, in the near-wellbore region or in one or more strata ofthe formation. Such applications are those in which without a filtercake fluid would leak off into porous rock at an undesirable rate duringa well treatment. Such treatments include drilling, drill-in,completion, stimulation (for example, hydraulic fracturing or matrixdissolution), sand control (for example gravel packing, frac-packing,and sand consolidation), diversion, scale control, water control, andothers. Typically, after these treatments have been completed thecontinued presence of the filter cake is undesirable or unacceptable.

Solid, insoluble, materials (that may be called fluid loss additives andfilter cake components) are typically added to the fluids used in thesetreatments to form the filter cakes, although sometimes soluble (or atleast highly dispersed) components of the fluids (such as polymers orcrosslinked polymers) may form the filter cakes. Removal of the filtercake is typically accomplished either by a mechanical means (scraping,jetting, or the like), by subsequent addition of a fluid containing anagent (such as an acid, a base, or an enzyme) that dissolves at least aportion of the filter cake, or by manipulation of the physical state ofthe filter cake (by emulsion inversion, for example). These removalmethods usually require a tool or addition of another fluid (for exampleto change the pH or to add a chemical). This can sometimes be done inthe wellbore but normally cannot be done in a proppant or gravel pack.Sometimes the operator may rely on the flow of produced fluids (whichwill be in the opposite direction from the flow of the fluid when thefilter cake was laid down) to loosen the filter cake or to dissolve thefilter cake (for example if it is a soluble salt). However, thesemethods require fluid flow and often result in slow or incomplete filtercake removal. Sometimes a breaker can be incorporated in the filter cakebut these must normally be delayed (for example by esterification orencapsulation) and they are often expensive and/or difficult to placeand/or difficult to trigger.

There is a need for a new composition and method in which a filter cakeis formed from at least two components, one of which slowly reacts withwater, and the second of which reacts with a reaction product of thefirst to destroy the filter cake spontaneously.

SUMMARY OF INVENTION

One embodiment is an oilfield treatment composition including first asolid that is one or more of lactide, glycolide, polylactic acid,polyglycolic acid, copolymers of polylactic acid and polyglycolic acid,copolymers of glycolic acid with other hydroxy-, carboxylic acid-, orhydroxycarboxylic acid-containing moieties, copolymers of lactic acidwith other hydroxy-, carboxylic acid-, or hydroxycarboxylicacid-containing moieties, and mixtures of the preceding, and second asolid that reacts with an acid. We will call the former a “solidacid-precursor” and the latter a “solid acid-reactive material”. Inanother embodiment, the solid acid-reactive material is capable of atleast partially dissolving in an aqueous fluid. In yet anotherembodiment, the solid acid-reactive material promotes the formation ofacid from the solid acid-precursor. In another embodiment of theInvention, solid particles or fibers or other shapes of the solidacid-precursors of the Invention are formed that include othermaterials, useful in oilfield treatments, for example solidacid-reactive materials such as calcium carbonate, aluminum hydroxide,magnesium oxide, calcium oxalate, calcium phosphate, aluminummetaphosphate, sodium zinc potassium polyphosphate glass, and sodiumcalcium magnesium polyphosphate glass. The solid acid-precursor in theoilfield treatment composition, including an embodiment in which it ismixed with or contains other materials, may be coated or encapsulated.

Methods of the Invention include incorporation of solid acid-precursorsand acid-reactive materials in treatment fluids to form filter cakes indrilling, drill-in and completion treatments, in hydraulic fracturingtreatments, in diversion treatments, in scale control treatments, inwater control treatments, in matrix dissolution treatments, in sandconsolidation treatments, in frac-packing treatments, and in gravelpacking treatments such that delayed acid generation occurs to delay atleast part of the filter cake after the drilling, completion,fracturing, diversion or sand control treatment. Other embodimentsinclude using the solid acid-precursors and the solid acid-reactivematerials in combination as components of fluid loss additives thatgenerate acid, after their use, to destroy some or all of the fluid lossadditive. Other embodiments include using the solid acid-precursors andsolid acid-reactive materials in combination as components of drillingfluids, drill-in fluids, completion fluids, diversion fluids, andstimulation fluids such that the solid acid-precursors form part of thefilter cake and then form acids in the filter cake to react with thesolid acid-reactive materials to destroy some or all of the filter cakeafter a suitable delay.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows the ability of various organic acids to dissolve calcite.

DETAILED DESCRIPTION

Excellent sources of acid that can be generated downhole when and whereit is needed are solid cyclic dimers, or solid polymers, of certainorganic acids, that hydrolyze under known and controllable conditions oftemperature, time and pH to form the organic acids. We will call thesesolid materials “acid-precursors” and we will call the formation of aciddownhole “delayed acid generation”. One example of a suitable solidacid-precursor is the solid cyclic dimer of lactic acid (known as“lactide”), which has a melting point of 95 to 125° C., (depending uponthe optical activity). Another is a polymer of lactic acid, (sometimescalled a polylactic acid (or “PLA”), or a polylactate, or apolylactide). Another example is the solid cyclic dimer of gylycolicacid (known as “glycolide”), which has a melting point of about 86° C.Yet another example is a polymer of glycolic acid (hydroxyacetic acid),also known as polyglycolic acid (“PGA”), or polyglycolide. Anotherexample is a copolymer of lactic acid and glycolic acid. These polymersand copolymers are polyesters.

Cargill Dow, Minnetonka, Minn., USA, produces the solid cyclic lacticacid dimer called “lactide” and from it produces lactic acid polymers,or polylactates, with varying molecular weights and degrees ofcrystallinity, under the generic trade name NATUREWORKS™PLA. The PLA'scurrently available from Cargill Dow have molecular weights of up toabout 100,000, although any polylactide (made by any process by anymanufacturer) and any molecular weight material of any degree ofcrystallinity may be used in the embodiments of the Invention. The PLApolymers are solids at room temperature and are hydrolyzed by water toform lactic acid. Those available from Cargill Dow typically havecrystalline melt temperatures of from about 120 to about 170° C., butothers are obtainable. Poly(d,l-lactide) is available from Bio-Invigor,Beijing and Taiwan, with molecular weights of up to 500,000. Bio-Invigoralso supplies polyglycolic acid (also known as polyglycolide) andvarious copolymers of lactic acid and glycolic acid, often called“polyglactin” or poly(lactide-co-glycolide). The rates of the hydrolysisreactions of all these materials are governed by the molecular weight,the crystallinity (the ratio of crystalline to amorphous material), thephysical form (size and shape of the solid), and in the case ofpolylactide, the amounts of the two optical isomers. (The naturallyoccurring I-lactide forms partially crystalline polymers; syntheticdl-lactide forms amorphous polymers.) Amorphous regions are moresusceptible to hydrolysis than crystalline regions. Lower molecularweight, less crystallinity and greater surface-to-mass ratio all resultin faster hydrolysis. Hydrolysis is accelerated by increasing thetemperature, by adding acid or base, or by adding a material that reactswith the hydrolysis product(s).

Homopolymers can be more crystalline; copolymers tend to be amorphousunless they are block copolymers. The extent of the crystallinity can becontrolled by the manufacturing method for homopolymers and by themanufacturing method and the ratio and distribution of lactide andglycolide for the copolymers. Polyglycolide can be made in a porousform. Some of the polymers dissolve very slowly in water before theyhydrolyze.

Other materials suitable as solid acid-precursors are all those polymersof hydroxyacetic acid (glycolic acid) with itself or other hydroxy-,carboxylic acid-, or hydroxycarboxylic acid-containing moietiesdescribed in U.S. Pat. Nos. 4,848,467; 4,957,165; and 4,986,355.

In many oilfield applications, fluid loss additives and filter cakes areneeded during a treatment, but after the treatment it is desirable thatthe fluid loss additive or filter cake be substantially gone. To makefluid loss additives and filter cake components, acid-soluble oracid-reactive materials, such as but not limited to magnesia, aluminumhydroxide, calcite, calcium oxalate, calcium phosphate, aluminummetaphosphate, sodium zinc potassium polyphosphate glass, and sodiumcalcium magnesium polyphosphate glass are mixed with or incorporatedinto, solid acid-precursors, such as cyclic ester dimers of lactic acidor glycolic acid or homopolymers or copolymers of lactic acid orglycolic acid. These fluid loss additives and filter cake components areadded to fluids injected into the subsurface in oilfield operations. Atleast a portion of the solid acid-precursors slowly hydrolyzes atcontrollable rates to release acids at pre-selected locations and times.The acids then react with and dissolve at least a portion of theacid-reactive materials. The result is that at least a portion of boththe solid acid-precursor and the acid-reactive solid material dissolve.We will term this “self-destruction” of the mixture. This feature ofthese materials is used to improve many oilfield treatments. Preferablymost or all of the solid material initially added is no longer presentat the end of the treatments. It is not necessary either for all of thesolid acid-precursor to hydrolyze or for all of the solid acid-reactivematerial to dissolve. It is necessary only that a sufficient amount ofeither no longer be a solid portion of the filter cake so that thefilter cake no longer forms a deleterious barrier to fluid flow.

Mixtures of one or more solid acid-precursors and one or more solidacid-reactive materials may be purely physical mixtures of separateparticles of the separate components. The mixtures may also bemanufactured such that one or more solid acid-precursors and one or moresolid acid-reactive materials is in each particle; this will be termed a“combined mixture”. This may be done, by non-limiting examples, bycoating the acid-reactive material with the solid acid-precursor, or byheating a physical mixture until the solid acid-precursor melts, mixingthoroughly, cooling, and comminuting. For example, it is common practicein industry to co-extrude polymers with mineral filler materials, suchas talc or carbonates, so that they have altered optical, thermal and/ormechanical properties. Such mixtures of polymers and solids are commonlyreferred to as “filled polymers”. When the solid acid-reactive materialis completely enclosed within the solid acid-precursor, the solidacid-reactive material may be water-soluble, for example boric acid orborax. In any case it is preferable for the distribution of thecomponents in the mixtures to be as uniform as possible. The relativeamounts of the components may be adjusted for the situation to controlthe solid acid-precursor hydrolysis rate and the rate and extent ofdissolution of the solid acid-reactive material. The most importantfactors will be the temperature at which the treatment will be carriedout, the composition of the aqueous fluid or fluids with which themixture will come into contact, and the time desired for dissolution ofthe mixture.

The solid acid-precursors or the mixtures of solid acid-precursors andsolid acid-reactive materials may be manufactured in various solidshapes, including, but not limited to fibers, beads, films, ribbons andplatelets. The solid acid-precursors or the mixtures of solidacid-precursors and solid acid-reactive materials may be coated to slowthe hydrolysis further. Suitable coatings include polycaprolate (acopolymer of glycolide and epsilon-caprolactone), and calcium stearate,both of which are hydrophobic. Polycaprolate itself slowly hydrolyzes.Generating a hydrophobic layer on the surface of the solidacid-precursors or the mixtures of solid acid-precursors and solidacid-reactive materials by any means delays the hydrolysis. Note thatcoating here may refer to encapsulation or simply to changing thesurface by chemical reaction or by forming or adding a thin film ofanother material. Another suitable method of delaying the hydrolysis ofacid-precursor, and the release of acid, is to suspend the solidacid-precursor, optionally with a hydrophobic coating, in an oil or inthe oil phase of an emulsion. The hydrolysis and acid release do notoccur until water contacts the solid acid-precursor.

An advantage of the composition and method embodiments of the Inventionis that, for a given oilfield treatment, the appropriate solidacid-precursor and solid acid-reactive material may be selected readilyfrom among many available materials. The rate of acid generation from aparticular solid acid-precursor or a particular mixture of a solidacid-precursor and a solid acid-reactive material, having a particularchemical and physical make-up, including a coating if present, at aparticular temperature and in contact with a fluid or fluids of aparticular composition (for example pH and the concentration and natureof other components, especially electrolytes), is readily determined bya simple experiment: exposing the acid-precursor to the fluid or fluidsunder treatment conditions and monitoring the release of acid. The rateof solid acid-reactive material dissolution is governed by similarfactors (such as by the choice of solid acid-reactive material, theratio of materials, the particle size, calcining and coating of solidacid-reactive material) and may readily and easily be determined bysimilar experiments. Naturally, a solid acid-precursor is selected thata) generates acid at the desired rate (after a suitable delay if needed)and b) is compatible with and does not interfere with the function ofother components of the fluid. An acid-reactive material is selectedthat dissolves in the evolving fluid at a suitable rate and iscompatible with the function of other components of the fluid. This isdone for all of the methods described below.

The mixture self-destructs in situ, that is, in the location where it isplaced. That location may be part of a suspension in a treatment fluidin the wellbore, in the perforations, in a gravel pack, or in afracture; or as a component of a filter cake on the walls of a wellboreor of a fracture; or in the pores of the formation itself. The mixturemay be used in carbonates and sandstones. If the formation issignificantly acid soluble, the amount of mixture, or the amount ofsolid acid-precursor in the mixtures, may be adjusted to account forconsumption of acid in reaction with the formation. In use, even thoughthe particles are intended to become part of a filter cake, they may endup in other places, where they are normally undesirable because theyimpede fluid flow, so in all locations self-destruction is desired.

The particle sizes of the individual components of the mixture may bethe same or different. The particle sizes of the individual componentsof the mixture or of the combined mixture, as they relate to the use asa fluid loss additive and as filter cake former components, dependprimarily upon the pore size distribution of the rock onto which thefilter cake is to be deposited and whether or not it is intended toeliminate or just to reduce fluid loss. Criteria for and methods of,choosing the optimal particle sizes or particle size distributions forconventional fluid loss additives and filter cake components are wellknown. Other particle sizes may be chosen for embodiments of the currentInvention; particle sizes or size distributions may be selected as acompromise between those that are optimal for fluid loss control orfilter cake formation and those that are optimal for self-destruction atthe desired time and rate. The rate of self-destruction can readily bemeasured in the laboratory in a given fluid at a given temperature.

A particular advantage of these materials is that the solidacid-precursors and the generated acids are non-toxic and arebiodegradable. The solid acid-precursors are often used asself-dissolving sutures.

The mixtures of solid acid-precursors and solid acid-reactive materialsare used as fluid loss additives, optionally in combination with othermaterials, as components of filter-cake forming compositions. Mixturesin the form of particulates, fibers, films, ribbons or other shapes areadded to the drilling, completion, or stimulation fluid to prevent orminimize leakoff during reservoir drilling, drill-in, or stimulationoperations but in the long term they dissolve and eventually clean upwithout an additional treatment step. Furthermore, if the mixture isformulated so that it generates acid in excess of that required todissolve the acid-reactive component, then the excess acid produced byhydrolysis stimulates the formation, if it contains acid-solublematerial, by etching either the surface of naturally occurring fracturesor the face of the formation at the wellbore. Such mixtures thatgenerate extra acid are particularly useful for drilling, “drill-in”,and stimulation operations carbonate reservoirs, especially in fracturedcarbonate reservoirs. Also, an appropriate amount of buffer may be addedto the fluid or to the particles to counteract the effects of acid beinggenerated by premature hydrolysis of the solid acid-precursor.

Similarly, a self-destructing fluid leak-off and filter cake formingadditive is made for drilling, completions, wellbore intervention andfracturing operations. A self-destructing drill-in fluid includes amixture of the solid acid-precursor and an acid-soluble particulatematerial, such as but not limited to CaCO₃, aluminum hydroxide, ormagnesis. This fluid creates a chemically metastable filtercake thatprevents fluid leakoff and formation damage during the drilling processbut readily cleans up over time. As the solid acid-precursor hydrolyzesit forms an acid that attacks the carbonate or other particles and,since the solid acid-precursor and carbonates or other materials areintermingled during deposition, the cleanup process is uniform andextensive. In particularly preferred embodiments, the acid-solublematerial has a high solubility in the in situ generated acid, that is, agiven amount of the acid dissolves a large amount of the acid-solublematerial.

In hydraulic fracturing, frac-packing, and gravel packing embodiments,the solid acid-precursor may be added in the pad, throughout thetreatment or to only some of the proppant or gravel stages. The solidacid-precursor or mixture may be a fiber in any of these uses and willretard flowback of proppant or gravel, and/or of fines if they arepresent, until the solid-acid-precursor hydrolyzes and the mixturedissolves. A self-destructing fluid loss additive and filter cake isparticularly useful in hydraulic fracturing, frac-packing, and gravelpacking because mechanical removal methods are impossible and methodsinvolving contacting the fluid loss additive and filter cake with anadditional fluid are not practical. For example, calcite is known to bean excellent fluid loss additive, but calcite is not soluble in water,even at 150° C. Calcite has been used for years in drilling fluids toform filter cakes that are subsequently removed with acid. Furthermore,solid acid-precursors such as polyglycolic acid soften and deform athigh temperatures, whereas particles of materials such as magnesiumoxide are hard. The deformation of the softened polyglycolic acid trapsthe magnesium oxide and makes it an even better fluid loss additive andfilter cake former.

There are a number of composition embodiments of the Invention. In thesimplest embodiment, sized particles, beads, fibers, platelets orribbons (or other shapes) of solid acid-precursor are mixed with sizedparticles of calcium carbonate in a drill-in fluid. It is also withinthe scope of the Invention to manufacture particles that contain boththe solid acid-precursor and the acid-soluble particulate material, forexample to co-extrude (and optionally then to comminute) mixtures ofcalcium carbonate and solid acid-precursor in particles, fibers,platelets or ribbons that are used for this function. Calcium carbonateor other solid acid-reactive material coated with solid acid-precursormay also be used. In these uses, the tightness of the packing of theparticles in the filtercake may also be used to control the rates ofgeneration of acid and dissolution of particles by affecting localconcentrations of reactants and products, convection, and other factors.

Another advantage to the use the mixtures of the Invention in fluid lossadditives and filter cakes is that the acid generated in theself-destruction process may function as a breaker for polymeric orviscoelastic surfactant viscosifying agents. Acids are known to damageor destroy synthetic thetic polymers and biopolymers used to viscositydrilling, completion and stimulation fluids. Acids are also known todamage or destroy either the micelle/vesicle structures formed byviscoelastic surfactants or, in some cases, the surfactants themselves.

When solid acid-precursors or mixtures of solid acid-precursors andsolid acid-reactive materials are used in fluids in such treatments asdrilling, drill-in, completion, stimulation (for example, hydraulicfracturing or matrix dissolution), sand control (for example gravelpacking, frac-packing, and consolidation), diversion, and others, thesolid acid-precursor or mixture of solid acid-precursor and solidacid-reactive material are initially inert to the other components ofthe fluids, so the other fluids may be prepared and used in the usualway. Normally, such fluids already contain a fluid loss additive andfilter cake former, so the solid acid-precursor or mixture of solidacid-precursor and solid acid-reactive material replace some or all ofthe fluid loss additive and filter cake former that would otherwise havebeen used. In many cases, if the fluid contains a component that wouldaffect or be affected by the solid acid-precursor or mixture of solidacid-precursor and solid acid-reactive material (such as a buffer,another acid-reactive material, or a viscosifier that forms or isincorporated in filter cakes), either the amount or nature of the solidacid-precursor or mixture of solid acid-precursor and solidacid-reactive material or the amount or nature of the interfering orinterfered-with component may be adjusted to compensate for theinteraction. This may readily be determined by simple laboratoryexperiments.

Although the compositions and method embodiments of the Invention aredescribed in terms of producing wells for oil and/or gas, thecompositions and methods have other uses, for example they may also beused in injection wells (such as for enhanced recovery or for storage ordisposal) or in production wells for other fluids such as carbon dioxideor water.

EXAMPLE 1

Lactic acid is not as commonly used as an acid in oilfield treatments asare formic, acetic and citric acids. Tests were run to determine thecapacity of lactic acid in the dissolution of calcite at 82° C. FIG. 1shows the concentration of calcite in ppm dissolved by reagent gradelactic acid as a function of weight percent acid in water. Lactic acidhas a capacity for dissolving calcite that is similar to acetic acid orformic acid, and much higher than citric acid. These tests demonstratethat lactic acid generated from a lactate polymer is effective fordissolution of calcium carbonate.

EXAMPLE 2

Experiments were performed (Table 1) to evaluate the hydrolysis rate ofPLA and to compare the hydrolysis rates of PLA with and without addedcalcite. The PLA was NATUREWORKS™ PLA Polylactide Resin 4042D, apolymerized mixture of D- and L-lactic acid, available from Cargill Dow,Minnetonka, Minn., USA. The material was used as approximately 4 mmdiameter beads. The calcite was reagent grade powder. 45.04 Grams PLAand 20 grams calcite, when used, were added to 500 ml distilled water.The time shown is the time for 100% hydrolysis.

TABLE 1 Compo- sition 121° C. 135° C. 149° C. PLA Dissolves in greaterDissolves in greater Dissolves in less than 2 hours than 2 hours than 2hours PLA + Dissolves in greater Dissolves in less than Dissolves inless Calcite than 2 hours 30 2 hours 30 minutes than 45 minutes minutesCalcite Insoluble Insoluble Insoluble

These results show that this solid acid-precursor hydrolyses anddissolves at a rate suitable for use as a self-destructive fluid lossadditive and filter cake former. Furthermore, calcite, which isinsoluble in water under these conditions, accelerates the rate of PLAhydrolysis and is itself dissolved in the generated acid.

EXAMPLE 3

Experiments were run to determine the suitability of various materialsas fluid loss additives. Experimental conditions and results are shownin Table 2. Berea sandstone cores (2.54 cm long and 2.54 cm in diameter)were mounted in an API static fluid loss cell. Cores were flushed with2% KCl brine, heated to the indicated temperature, and the permeabilityto the brine was determined at a flow rate of 5 ml/min. Then theindicated fluid was injected at a constant pressure of 6.895 MPa. Theeffluent fluid was determined with a balance and recorded as a functionof time. Leak-off was characterized in two ways: the “spurt”, which wasthe initial rapid leak-off of fluid before a filter cake barrier wasformed on the core face (indicated by the grams fluid leaked off in thefirst 30 seconds), and, “wall”, which was the subsequent leak-off thatoccurred even after a filter cake was formed (indicated by the grams perminute of fluid leaked off between 15 and 30 minutes).

All concentrations shown in Table 2 are in weight percent. Thesurfactant used in all experiments was obtained from the supplier(Rhodia, Inc. Cranbury, N.J., U. S. A.) as Mirataine BET-E-40; itcontains 40% active ingredient (erucylamidopropyl betaine), with theremainder being substantially water, sodium chloride, and isopropanol.The MgO used was MagChem 35, obtained from Martin Marietta MagnesiaSpecialties LLC, Baltimore, Md., USA. It has a mean particle size of 38microns. The PGA used was Dupont TLF 6267, described by the supplier asa crystalline material having a molecular weight of about 600 and a meanparticle size of about 8 to 15 microns. The Al(OH)₃ used was obtainedfrom Aldrich. It has a mean particle size of about 40 microns. The PGAand the solid acid-reactive materials were added as separate particles.The buffer used in Experiment 25 was sodium sesquicarbonate.

These data show that all the mixtures of PGA and magnesium oxide, sizedcalcium carbonate, or aluminum hydroxide are excellent fluid lossadditives and form filter cakes that very effectively reduce flowthrough these cores. (Without the additives, the flow through a 100 mDcore would be greater than 100 g in a 30 minute test.) The fluid lossadditive and filter cake former are effective at various totalconcentrations and ratios of solid acid-precursor to solid acid-reactivematerial, in cores having a broad range of quite high permeabilities,and at several temperatures. They reduce both the spurt and thesubsequent leak-off. Furthermore, when the composition of the Inventionis used, a lower concentration of surfactant may be required.

TABLE 2 Experiment Result Test ID Run Formulation Temp. Perm g/30 min“Sport” g “Wall” g/min 7598-11  1 3% Surfactant + 0.5% PGA + 0.4% MgO65.6 C. 167 mD 17 7598-113  2 3% Surfactant + 0.5% PGA + 0.4% MgO 65.6137 23 7598-114  3 3% Surfactant + 0.5% PGA + 0.4% MgO 65.6 152 11 20.29 7598-115  4 3% Surfactant + 0.5% PGA + 0.4% MgO 65.6 106 13 7598-17 5 6% Surfactant + 0.5% PGA + 0.4% MgO 65.6 235 12 7598-171  6 3%Surfactant + 0.5% PGA + 0.4% MgO 65.6 230 22 7598-172  7 3% Surfactant +0.5% PGA + 0.4% MgO 65.6 210 34 7598-18  8 6% Surfactant + 0.5% PGA +0.4% MgO 65.6 209 11 7598-19  9 6% Surfactant + 0.5% PGA + 0.4% MgO 65.6211 31 7598-21 10 6% Surfactant + 0.5% MgO 65.6 125 23 7.5 0.37 7598-23111 6% Surfactant + 0.2% PGA + 0.4% MgO 65.6  42 5.5 7598-232 12 6%Surfactant + 0.2% PGA + 0.4% MgO 65.6 171 6 2 0.088 7598-233 13 6%Surfactant + 0.2% PGA + 0.4% MgO 65.6 306 7 7598-24 14 3% Surfactant +0.2% PGA + 0.4% MgO 65.6 246 19 7598-25 15 6% Surfactant + 0.2% PGA +0.4% MgO 93.3  29 7 7598-251 16 6% Surfactant + 0.2% PGA + 0.4% MgO 93.3126 7.5 7598-252 17 6% Surfactant + 0.2% PGA + 0.4% MgO 93.3 299 9.57598-28 18 3% Surfactant + 0.2% PGA + 0.4% MgO 93.3  51 17 7598-281 193% Surfactaat + 0.2% PGA + 0.4% MgO 93.3 119 18 7598-29 20 3%Surfactant + 0.2% PGA + 0.4% MgO 93.3 300 20 7598-31A 21 3% Surfactant +0.2% PGA + 0.4% CaCO3 (2 micron) 65.6  48 29 7.5 0.52 7598-31B 22 3%Surfactant + 0.2% PGA + 0.4% CaCO3 (10 micron) 65.6  40 26 7598-31C 236% Surfactant + 0.2% PGA + 0.4% CaCO3 (10 micron) 65.6  43 11 2.5 0.217598-31D 24 3% Surfactaut + 0.2% PGA + 0.4% CaCO3 (2 micron) + 0.15% MgO65.6 107 31 7598-39B 25 3% Surfactant + 0.2% PGA + 0.4% AI(OH)3 + 0.2%Buffer 65.6 117 34 6 0.64 7598-39C 26 3% Surfactant + 0.2% PGA + 0.4%Al(OH)3 65.6 128 74 8 1.25

1. An oilfield treatment composition comprising a physical mixture ofseparate particles of separate solid acid-precursor and solidacid-reactive material components; wherein the solid acid-precursor isselected from the group consisting of lactide, polylactic acid, andmixtures thereof; and wherein the solid acid-reactive is a materialsubstantially insoluble in water, boric acid or borax; provided that themixture of the solid acid-precursor and the solid acid-reactive materialis not a combined mixture.
 2. The composition of claim 1 wherein thesolid acid-reactive material is selected from the group consisting ofmagnesium hydroxide, calcium carbonate, aluminum hydroxide, calciumoxalate, calcium phosphate, aluminum metaphosphate, sodium zincpotassium polyphosphate glass, and sodium calcium magnesiumpolyphosphate glass.
 3. The composition of claim 1 wherein the solidacid-precursor is coated with a hydrolysis-delaying material.
 4. Thecomposition of claim 1 wherein the mixture of separate particles of asolid acid-precursor and a solid acid-reactive material is capable offorming a self-destructing filter cake.
 5. The composition of claim 1wherein the solid acid-reactive material is incorporated in an amountsuch that when the mixture contacts water, hydrolysis of the solidacid-precursor is accelerated, and wherein the solid acid-reactivematerial is dissolved by the acid generated due to the hydrolysis of thesolid acid-precursor.